The present invention relates to oil wells and particularly to wells that are produced by rod pumping. The term `rod pumping` refers to a pumping system in which a reciprocating pump located at the bottom of the well is actuated by a string of rods. The rods are reciprocated by a pumping unit located at the surface. The unit may be of the predominant beam type or any other type that reciprocates the rod string. A beam pumping unit utilizes a walking beam pivotally mounted on a Samson post with one end of the beam being attached to the rods and with the beam being reciprocated by a drive unit. The drive unit consists of a prime mover connected to a reduction unit that drives a crank to reciprocate the walking beam.
The downhole pump consists of a barrel attached to (or part of) the production tubing string that is anchored to the well casing. A plunger reciprocates in the barrel which is attached to the end of the rod string. The barrel is provided with a standing valve, and the plunger is provided with a traveling valve. On the down stroke, the traveling valve opens and the standing valve closes, allowing the fluid in the barrel to pass through the plunger. On the up stroke the traveling valve closes allowing the plunger to lift fluid to the surface while the standing valve opens and the plunger draws more fluid from the well into the barrel.
Pumping systems are normally sized so that they can produce essentially all of the fluid from the well using controllers which alternately pump the well or shut it down when necessary to allow more fluid to enter the casing. The controllers can be simple clock timers that start and stop the pumping unit in response to a set program or controllers that control the pumping unit in response to some measured characteristics of the pumping system.
Controllers that control the pumping unit in response to measured pumping characteristics are designed to shut the pumping unit down when the well has pumped off. This saves energy and prevents damage to the pumping system. The term pumped-off is used to describe the condition where the fluid level in the well is not sufficient to completely fill the pump barrel on the upstroke. On the next downstroke the plunger will impact the fluid in the incompletely filled barrel and send shock waves through the rod string and other components of the pumping system. This can cause harm to the pumping system such as broken rods or damage to the drive unit or downhole pump. All pump-off controllers are designed to detect when a well pumps off and to shut the well down.
In the Applicant's prior U.S. Pat. No. 3,951,209, there is described a controller that measures at the surface both the load on the rod string and the displacement of the rod string. From these measurements, one can obtain a surface dynamometer card and the area of the card will be the power input to the rod string. Since the pumping system will be lifting less fluid when the well pumps off, the power input to the rod string will also decrease. The decrease in power will result in a decrease in the area of the surface dynamometer card. This decrease in area is used as an indication of a pump-off condition and the pumping unit is shut down. U.S. Pat. No. 4,015,469 describes an improvement of the '209 patent in which only a portion of the area of the surface card is considered. In particular, the '469 patent utilizes only the last part of the upstroke and the first part of the downstroke to detect pump-off. This is the portion of the surface card in which pump-off is usually shown.
Other methods have also been developed for detecting pump off. For example, U.S. Pat. No. 3,306,210 discloses a pump-off controller that monitors the load on the polished rod at a set position in the downstroke. Pump-off is detected when the load exceeds a preset level at that set position. U.S. Pat. No. 4,583,915 discloses a pump-off controller that monitors an area outside the surface dynamometer card. More particularly, the patent discloses monitoring an area between the minimum load line and the load line at the top of the stroke. Other pump-off controllers have monitored the electrical current drawn by the drive motor to detect pump-off.
The Applicant's U.S. Pat. No. 4,490,094 discloses a pump-off controller that monitors the instantaneous speed of revolution of the drive motor during a complete or portion of the cycle of the pumping unit. Pump off is sensed by calculating a motor power from measured speed which is less than motor power corresponding to a completely filled pump barrel. Both the surface load and position of the rod string can also be determined from the monitored instantaneous speed of the drive motor.